Analysts expect that average U.S. mortgage rates have declined since the U.K. vote to leave the European Union

Britain’s decision to leave the European Union could benefit a group thousands of miles away: U.S. homeowners.

Several lenders posted rates for 30-year, fixed-rate mortgages of about 3.5% on Monday, nearing a 3.5-year low, and analysts expect coming reports to show that average U.S. mortgage rates have decreased since the Brexit vote Thursday.

The main reason: Investors have flocked to the safety of U.S. Treasurys, pushing interest rates lower as riskier assets such as stocks tumbled. Mortgage rates tend to move up and down with 10-year Treasury rates, though the relationship isn’t perfect.

The yield on the 10-year Treasury note was 1.460% on Tuesday, down from 1.579% before the result of Thursday’s vote was announced and 2.273% at the end of 2015, according to Tradeweb.

Last Wednesday, the Mortgage Bankers Association reported that the average rate for a 30-year, fixed-rate mortgage was 3.76%. Housing-market analysts will be watching closely on Wednesday when the MBA releases its next weekly report.

If mortgage rates decline, that could be a boon both to prospective home buyers and to homeowners who qualify for refinancing. Refinancing could make sense for many people locked into rates of 4% or higher, according to mortgage experts.

Lenders are expecting a miniwave of refinancing in coming weeks. In all, 40% of borrowers have loans with a rate of 4.5% or higher, according to CoreLogic Inc., a real-estate analytics firm, meaning they could save about $90 a month on average by refinancing at 3.5%.

Lenders across the country said refinancing applications since Thursday are up between 10% to 40% compared with typical volume this time of year.

“Unless we see interest rates tick back up, I think we will see a boomlet” in refinancing activity, said Bob Walters, chief economist at Detroit-based Quicken Loans Inc., the country’s third-largest mortgage lender. Mr. Walters said Quicken is seeing refinancing applications up 30% to 40% compared with typical volume.

Because mortgage rates have been near multidecade lows for the past six years, most borrowers who could refinance already have done so. Those who held back largely did so because their home was still underwater or their credit was poor.

But strong home-price growth in the past several months has helped increase the pool of borrowers with equity in their homes who are eligible to refinance. About 13% of homeowners owe more than their home is worth, down from a high of 31% in 2012, according to Zillow, a real-estate research firm.

The Brexit vote is expected to usher in a period of economic uncertainty across Europe as companies and investors pause to weigh the impact and seek out safe investments such as government bonds. Economic turbulence could prompt the Federal Reserve to hold off on raising interest rates until next year, which would likely help keep mortgage rates low, economists said.

Economists said major global economic shocks tend to boost refinancing activity because homeowners have learned over the years to expect the possibility that rates will fall. Some economists and executives said they expect the impact will be longer lasting than that of other recent shocks, such as the Greek debt crisis several years ago or the Chinese stock-market tumble last August and this January.

“I think this will have some legs on it,” said Mr. Walters of Quicken Loans.

Scott Eggen, director of capital markets at Dallas-based PrimeLending, said the number of clients trying to lock in their rates at current levels is up 10% in June from May. He said that if rates remain low, he would expect an even bigger jump in activity.